The basic function of the auditory system is to allow listeners to detect signals. This needs to happen in both quiet and noisy environments. The ability to detect sounds in noisy environments is compromised in the elderly and the hearing impaired, even with assistive hearing devices. The goal of this project is to determine the neuronal mechanisms in the cochlear nucleus (CN) and inferior colliculus (IC) of primates that mediate detection in quiet and noisy environments. Detection in noise depends on auditory efferent pathways that are fully functional only in awake and behaving organisms. Primates are the choice for this study due to their phylogenetic similarity with humans, as well as the similarity of the primate and human brainstem, which are different from other mammals. Studies in auditory and other sensory system have shown that responses to signals in noise are enhanced by inhibitory influences that suppress the effects of the noise while allowing the signal response to be expressed. Local processing within the IC increases the inhibition, specially wideband inhibition, which cancels the responses to noise while maintaining responses to signals, and mediating detection, thereby rendering ICC responses better related to IC responses than CN. This project proposes to test these hypotheses by recoding single unit responses in the cochlear nucleus and inferior colliculus in awake and behaving primates, and relating them to simultaneously measured behavioral responses. Overall, this study will help determine some of the mechanisms that allow us to detect and perceive sounds in natural environments.